In many areas of the world, large natural gas deposits are found. These natural gas deposits, while constituting a valuable resource, have little value in the remote areas in which they are located. To utilize these resources effectively, the natural gas must be moved to a commercial market area. This is frequently accomplished by liquefying the natural gas to produce a liquefied natural gas (LNG), which is then transported by ship or the like to a market place. Once the LNG arrives at the marketplace, the LNG must be revaporized for use as a fuel, for delivery by pipeline and the like. Other cryogenic liquids frequently require revaporization after transportation also, but by far the largest demand for processes of this type is for cryogenic natural gas revaporization.
In many instances the natural gas is revaporized by the use of seawater as a heat exchange medium, by direct-fired heaters and the like. Each of these methods is subject to certain disadvantages. For instance, there are concerns about the use of seawater for environmental and other reasons. Further, seawater in many instances is prone to contaminate heat exchange surfaces over periods of time. The use of direct-fired heaters requires the consumption of a portion of the product for heating to revaporize the remainder of the LNG.
While in some instances, air has been used as a heat exchange medium for LNG, the use of air has not been common because of the large heat transfer area required in the heat exchangers and because of the variable temperature of air during different seasons, during the day and night, and the like. Other disadvantages associated with the use of air relate to the formation of ice in the heat exchange vessels, the requirement for large amounts of air to heat the revaporized natural gas to a suitable temperature for delivery to a user or to a pipeline and the like. The use of such large volumes of air can require either excessively large heat exchange vessels or the use of excessive amounts of air, which may result in excessive expense for forced air equipment, high operating costs and the like. Accordingly, improved methods have continually been sought for more economically and effectively revaporizing cryogenic liquids.